The present invention relates to a data communication system and more particularly to a modem employed in such a system and used to transmit and receive compressed speech data.
Generally, high frequency (HF), radio voice and data communications have increased in importance in recent years, especially for military applications. As the HF band (2 to 30 MHZ) is more widely used, the need for reliable HF communication links increases. To improve the performance of voice and data modems in terms of bit error rates and immunity to interference and intercept, state of the art equalization, error correction and spread spectrum signal processing techniques must be employed.
Spread spectrum operation can be achieved by pseudo noise (PN) spreading in the modem or by frequency hopping (FH) the radio transmitter and receiver. Frequency hopping operation provides anti-jam or electronic counter counter measure operation (ECCM) capability while frequency hopping, pseudo noise spreading and the use of secure modem systems insure a low probability of intercept and a low probability of exploitation.
Essentially, reference is made to a U.S. Pat. No. 4,761,796 entitled "A High Frequency Spread Spectrum Communication System Termnal", Ser. No. 694,549, filed on Jan. 24, 1985 for J. G. Dunn et al. This application describes in detail a spread spectrum communication system or a modem terminal which essentially consists of a voice subsystem couple to a modem subsystem which is coupled to an RF subsystem. The modem subsystem uses state of the art equalization techniques and spread spectrum signal processing. The terminal of the communications system provides for 4800 bits per second and 2400 bits per second data communication on 6 KHZ and 3 KHZ high frequency channels with frequency hopping by the radio frequency RF transmitter/receiver contained in the RF subsystem. Frequency hopping combined with decision feedback equalization, linear intersymbol interference cancellation, error correction and interleaving in the modem subsystem gives frequency diversity and anti-jam capability. Very low bit rate voice communication in the voice subsystem is also provided by the high frequency communication system terminal. In this mode of operation the modem subsystem uses PN spreading to employ the full bandwidth that is available and to provide additional anti-jam capability. The frequency hopping/PN technique also gives low probability of intercept capability if low transmission powers are used.
As one can ascertain from reference to the above noted application, spread spectrum communication systems have been used in a variety of fields. In such a communication system the transmitted bandwidth is much greater than the bandwidth or rate of the information to be transmitted. The carrier signal is modulated by some other function to widen or spread the bandwidth for transmission.
The receive signal is remapped into the original information bandwidth to reproduce a desired signal. The spread spectrum communication system has many useful advantages, as is well known in the prior art. Such systems can be categorized into a direct sequence system, a frequency hopping system, a time hopping system and a hybrid system which is a proper combination of the systems just mentioned. Of these systems the frequency hopping system is frequently used in the field of mobile communication systems with a low traffic volume for a number of stations. Frequency hopping systems can be employed in satellite communication systems and scatter-type communications system where a fading environment is present.
In the frequency hopping system a carrier frequency is shifted or jumped in discrete increments in a pattern dictated by a prepared code or sequence, for instance, a PN code (pseudo noise), M sequence codes, Gold codes and the like in synchronism with a change in the state of the codes. The resulting consecutive and time sequential frequency pattern is called a hopping pattern and the duration of each hopping frequency is called a chip. The transmitted information is embedded in the codes or embedded in each frequency of the carrier wave by a so-called FSK (frequency shift keyed) or PSK (phase shift keyed) modulation. The information signal thus spread spectrum modulated can be reproduced at the receiver. Generally speaking, in reproducing the information signal at the receiver a synchronization acquisition process is first performed in which the code pattern provided in the receiver is made accurately coincident with the code pattern generated in the transmitter in time position. Then the spread spectrum signal is despread and thereafter a well known demodulation is performed to extract the desired information.
More particularly, a local reference signal of a frequency correspondingly determined by the same code pattern as that in the transmitter for every chip and the received signals are mixed in a mixer in order to perform a correlation (despreading) process for converting the spread spectrum signal into the signal having a frequency bandwidth wide enough to extract the information. This system is described in detail in "Spread Spectrum Systems" by R. C. Dixon published by John Wiley and Sons, Inc. (1976). Following this spreading process the desired information is extracted by usual demodulation techniques.
In general, in order to implement a frequency hopping communication system the terminal to be described is interfaced to a radio that is capable of frequency hopping the packets provided by the terminal. Many examples of such radios exist. As will be explained, the system according to this invention employs two types of frequency hopping modes. The first type is a Slow Hopping or High Data Rate (HDR) mode, which operates in 2400 voice, 2400 data or rekey modes. A Fast Hopping, or Low Data Rate (LDR) mode operates in 400 or rekey modes. The 2400 Voice Mode operates in conjunction with a standard voice processor utilizing linear predictive coding (LPC) which is well known in the prior art. The 400 Voice Mode has an additional voice processor function which converts standard 2400 LPC to 400 bits per second voice data.
As indicated, the modem to be described is a secure digital voice and data terminal. It is designed to interface with HF radios to provide a complete communication system. The terminal is particularly designed to interface with various military receivers, such as those manufactured by the Hughes Aircraft Company of California. Such radios implement frequency hopping modes which are compatible with the enhanced modes, as will be further described and which are available by the use of this terminal. The radio provides for the generation of the frequency hopping pattern and the actual shift in the carrier frequency. It also provides for the initial synchronization acquisition processes.
In any event, as one can further understand, jammer resistance is improved by increasing the rate of frequency hopping. A method of jamming when the frequency hopping is slow is for the jammer to repeat back the communication systems own signal. Thus, as one can ascertain, the jammer may be a communications transmitter and receiver combination whereby it is capable of repeating back the generated signal from the communication systems. If the delay of the repeat back interference is short compared to the packet duration, it will interfere with the communication signal. Thus, it is desirable to use as high a rate of frequency hopping as feasible to minimize the possibility of this type of jamming.
In any event, higher data rates can be transmitted by coherent, multi-phase modulation but reception from an HF channel requires equalization to remove intersymbol interference caused by multi-path spread of the received data symbols. Equalization, in turn, requires measurement of the channel response by means of a transmitted training signal. In a frequency hopping system each packet experiences a different channel response because it is at a different carrier frequency. This means that a training signal must be transmitted during each packet. The duration of the training signal must be greater than the duration of the multi-path spread.
The efficiency of the overall data signal is determined by the fraction of the packet which is used for training data. Thus, for good efficiency, the packet duration for a high data rate system must be much longer than the multi-path spread. Thus a high data rate system, using slow frequency hopping, is more susceptible to jamming.
On the other hand, fast frequency hopping systems are limited to incoherent modulation without any training signals which inherently provide a lower data rate. In a system which uses a training signal it is desirable that the data pattern be randomized by PN coding or the like so that it is different for each frequency hop. If it is the same for every frequency hop the system is easier to jam because the fixed training sequence can be transmitted as a jamming signal. However, it is more difficult to use a training signal which is different for each frequency hop.
It is therefore an object of the present invention to provide an improved high frequency communication system which system employs improved anti-jamming techniques.
It is a further object of the present invention to provide a high frequency anti-jam communication system which provides anti-jam protection by means of frequency hopping and error correction coding.
It is a further object of the present invention to provide a high frequency anti-jam communication system terminal which provides unique frequency hopping modes.